1. Technical Field of the Invention
The present invention relates to a foil bearing which includes a thin-film-shaped foil disposed on the outer periphery of a rotary shaft and an outer ring disposed on the outer periphery of the foil, and which rotatably supports the rotary shaft by forming a fluid lubrication membrane between the rotating rotary shaft and the foil.
2. Description of the Related Art
Foil bearings are used in turbo compressors, turbo chargers, micro gas turbines, and the like due to the benefit that oil is not required and that a fast rotating rotary shaft can be supported by using peripheral gas (for example, air) as an operation fluid.
An existing foil bearing is disclosed in, for example, the unpublished patent of Patent Document 1. In addition, the technology related to the invention is disclosed in Patent Document 2.
Further, the foil bearing is applied to a thrust bearing for supporting a fast rotating rotary shaft in the thrust direction (for example, Non-Patent Document 1).
In Patent Document 1, as shown in FIG. 1, the foil bearing includes a rotary shaft 51, a foil 52 which is wound on the outer periphery of the rotary shaft 51 as multiple layers, and an outer ring 53 which is provided on the outer periphery of the foil 52. A pair of stoppers 54 is provided in both ends of the outer ring 53 so that the foil 52 does not protrude in the axial direction.
Since a fluid lubrication membrane is formed between the rotary shaft 51 and the foil 52 when rotating the rotary shaft 51, the foil bearing 50 rotatably supports the rotary shaft 51.
The foil 52 includes a top foil of an inner peripheral side 52a and a back foil 52b of an outer peripheral side. As the back foil used in the past was a bump foil formed in a wave sheet shape as shown in FIG. 2.
Further, in the top foil 52a and the bump foil 52b, one ends are fixed to the outer ring 53 so as to prevent the separation thereof, and the other ends are formed as free ends.
The fluid lubrication membrane is formed between the top foil 52a and the rotary shaft 51 when rotating the rotary shaft 51, and the load acting on the rotary shaft 51 presses and deforms the top foil 52a via the fluid lubrication membrane. However, since the top foil is elastically supported by the bump foil, the deformation amount is restricted to an appropriate degree. In addition, since the bump foil (wave sheet) is deformed to widen the pitch of the individual wave when the top foil is deformed, sliding occurs between the top foil and the bump foil. However, since vibration energy is dissipated due to friction caused by the slide when the rotary shaft is vibrated, there is an advantage that the vibration (axial vibration) of the rotary shaft is suppressed. In Patent Document 2, as an alternative means for the function of the bump foil, used is a plurality of spring foils to stabilize the axial vibration.    [Non-Patent Document 1]    THERMAL MANAGEMENT PHENOMENA IN FOIL GAS THRUST BEARINGS, Proceedings of GT2006, ASME Turbo Expo 2006: Power for Land, Sea and Air, May 8-11, 2006, Barcelona, Spain    [Patent Document 1]    Japanese Patent Application No. 2007-209976, “FOIL BEARING DEVICE”, ‘UNPUBLISHED’    [Patent Document 2]    Japanese Patent Application Laid-Open No. 2004-270904, “FOIL FLUID BEARING”
In the aforementioned foil bearing 50, as shown in FIG. 3, since one end (the left end in the drawing) of the wave-sheet-shaped bump foil is fixed to the outer ring, when the bump foil is pressed from the top foil, each of the waves slides in the direction toward the free end (the right direction in the drawing) while widening the pitch thereof. However, since the slide is restrained as it approaches the fixed end, there are problems in that the estimation of the deformation stiffness of the wave, that is, the support stiffness of the top foil is difficult, and the desired support stiffness is not obtained. In addition, there are problems in that the slipping against the top foil becomes irregular (the slide becomes smaller as it approaches the fixed end, and becomes larger as it moves away from the fixed end) in accordance with the slide, and the estimation of vibration damping effect is difficult.